The present invention relates generally to circuit breaker systems. More particularly, the present invention relates to a circuit breaker system equipped to provide overcurrent protection via a magnetic latch.
Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload, a relatively high level short circuit, or a ground fault condition. To perform this function, circuit breakers presently include a switch unit and a trip unit. The switch unit is coupled to the electrical circuitry (i.e., lines and loads) such that it can open or close the electrical path of this electrical circuitry. The switch unit includes a pair of separable contacts per phase, a pivoting contact arm per phase, an operating mechanism, and an operating handle.
For each phase, a first contact of the pair of separable contacts is supported and moveable by the pivoting contact arm and a second contact is substantially stationary. All of the pivoting contact arms are coupled to the operating mechanism, and the operating mechanism is coupled to the operating handle. The operating handle is substantially disposed on the outside of the switch unit. In this manner, the operating mechanism can simultaneously actuate the pivoting contact arms, thereby either engaging or disengaging the pairs of separable contacts, in response to manual manipulation of the operating handle (i.e., a switch). Thus, in the overcurrent condition, all the pairs of separable contacts are disengaged or tripped (i.e., opening the electrical circuitry), and when the overcurrent condition is no longer present, the circuit breaker can be reset such that all the pairs of separable contacts are engaged (i.e., closing the electrical circuitry).
In addition to manual overcurrent protection via the operating handle, automatic overcurrent protection is also provided via the trip unit. The trip unit, coupled to the switch unit, senses the electrical circuitry for the overcurrent condition and automatically trips the circuit breaker. When the overcurrent condition is sensed, a tripping mechanism included in the trip unit actuates the operating mechanism, thereby disengaging the first contact from the second contact for each phase. Typically, the operating handle is coupled to the operating mechanism such that when the tripping mechanism actuates the operating mechanism to separate the contacts, the operating handle also moves to a tripped position.
There are two types of trip units: a thermal-magnetic trip unit and an electronic trip unit. The thermal-magnetic trip unit is a mechanical system that utilizes thermal or magnetic field changes in one or more components within the trip unit to sense the overcurrent condition. The electronic trip unit is an electronic system that includes, among others, circuitry, current transformers, an electromechanical interface unit, and solid-state devices to sense the overcurrent condition. Presently, the electromechanical interface unit in the electronic trip unit requires dedicated components and/or circuitry to provide a relatively high current to actuate the electromechanical interface unit or to selectively isolate the electromechanical interface unit from other parts of the electronic trip unit.
Thus, there is a need for a circuit breaker capable of providing reliable electronic tripping. Further, there is a need for an electronic trip unit capable of providing a relatively low current actuation of an electromechanical interface unit included therein using a minimum of dedicated components and/or circuitry.